This is a fascinating concept with some interesting possibilities- Most students have a cell phone, it's affordable technology- Being able to leverage that ubiquitous technology to teach is a clever idea; however, the teacher using it should be savvy to the various ways that a cell phone can be misused in a classroom before they try it-
No matter what you specify and no matter how clearly you specify it, there's going to be a small but confused group that'll manage to pull out the most obscure file format known to humankind...